Introduction:
Trisomy 18, also called ‘Trisomy E’ or ‘Edward syndrome,’ is a chromosomal disorder with an extra copy of chromosome 18. Patients with trisomy 18 present with multiple birth defects and cognitive impairments. These babies will show slow intrauterine development. They either die before birth and do not survive more than a week or die soon after birth due to life-threatening medical conditions. They generally have a low birth weight and are highly susceptible to certain cancers like Wilms tumor (kidney cancer) and hepatoblastoma (liver cancer). Unlike other genetic conditions, they are not inherited.
Who Identified Trisomy 18 Syndrome?
Mr.John Hilton Edwards et al. first described this genetic condition in 1960. They observed a neonate with several birth defects and malformations. From their observations, a third cell was found to be attached to a pair of 18 chromosomes. Later, Smith et al. confirmed that it was due to an extra copy of chromosome 18 responsible for trisomy 18.
How Common Is This Condition?
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It is a severe genetic condition that occurs in about one in 5000 to 6000 live births.
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Girls are more commonly affected than boys in a ratio of 4:1.
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They are often diagnosed during pregnancy and may end in miscarriage or stillbirth.
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The risk of trisomy 18 increases with the increase in maternal age.
What Are the Other Names for Trisomy 18?
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Edward syndrome.
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Complete trisomy 18.
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Trisomy 18.
What Are Chromosomes?
Each body cell contains a nucleus where the chromosome is present. In human beings, there are 46 chromosomes divided into 23 pairs. They carry the genetic material and the DNA in the cells. They instruct the body in the formation and functioning of the body. Of the 23 pairs, the 23rd pair is referred to as the sex chromosomes X and Y. For males, it is XY, and for females, it is XX. Each chromosome has a short arm designated by the letter p and a long arm designated as q.
What Causes Trisomy 18?
Trisomy 18 is caused due to the presence of an extra copy of chromosome 18q instead of two copies. This can be due to any abnormalities during conception (defective eggs or sperm) or while the baby is growing in the uterus itself.
What Are the Types of Trisomy 18?
Trisomy 18 is classified into three types. They are:
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Complete Trisomy - It is the most common form. About 94 % are complete trisomy. Here, there is an extra copy of chromosome 18 in each cell. This additional copy of the chromosome interferes with the development of the fetus causing growth abnormalities.
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Mosaic Trisomy - It is the second type and accounts for about five percent of the total trisomies. In the mosaic type, they do not have an extra copy in all the cells, whereas they are seen only in a few body cells.
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Partial Trisomy - It is seen in two percent of cases, and only a partial segment of the extra chromosome is present.
What Are the Symptoms of Edward Syndrome?
The clinical features vary in each individual, and there are more than 125 abnormalities reported so far. They can be categorized into two:
Prenatal Symptoms:
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The activity of the fetus is very minimal.
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The placenta is very small, and only a single artery is seen in the umbilical cord.
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Developmental delays.
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The amount of amniotic fluid surrounding the baby is high (polyhydramnios).
Postnatal Symptoms:
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Low birth weight and failure to thrive.
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Delayed developmental and intellectual abilities.
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Poor suckling ability and feeding habits
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Disabilities in the heart and kidney.
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Low muscle tone and hypotonia followed by hypertonia (stretching of muscles) during seizures or faint cries.
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Skeletal muscles and connective tissues are poorly developed.
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Craniofacial disabilities include small heads (microcephaly) that are long and narrow (dolichocephaly), small-sized mouth and jaws, widely spaced eyes and drooping eyelids, and cleft palate (incomplete closure of the roof of the mouth) and lips (grooves or slits seen in the lips).
In Severe Cases:
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They present with congenital heart and kidney defects.Heart defects account for about 90 % of premature deaths and stillbirths in babies.
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Respiratory abnormalities that are causing respiratory distress.
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Hernias (when the skin or organ pushes into the weaker part of the lower abdomen) are very common.
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Scoliosis (the curvature of the spine sideways).
How to Diagnose Trisomy 18?
The diagnosis of trisomy 18 is usually made during the prenatal period through routine screenings and ultrasounds. They are most often diagnosed after the 11th or 13th week.
1. Non-invasive Prenatal Screening (NIPT) - It is a blood examination that helps to detect the presence of trisomies 18 during the 10th week onwards.
2. Chorionic Villi Sampling - They are performed between the 10th and 13 weeks. Few cells of the placenta are collected and examined for genetic testing.
3. Amniocentesis - They are performed between 15 and 20 weeks. The amniotic fluid (the fluid surrounding the baby in the womb) is taken and assessed for the presence of any health conditions.
4. Nuchal Translucency - Increased nuchal translucency is suggestive of any congenital defects. It detects about 60 to 70 % of genetic conditions. However, they are combined with other ultrasounds to get 100 % results.
5. Maternal Serum Analysis Test - It is a simple blood investigation that helps detect specific genetic markers in the blood. They are:
- Alpha-Fetoprotein (AFP) - It is a protein produced in the developing fetus; higher values suggest neural tube defects.
- Human Chorionic Gonadotropin (hCG) - The hormone produced by the placenta during pregnancy.
- Unconjugated Estriol - The substance produced by the placenta. A low estriol level during the second trimester indicates the risk of fetal death.
What Are the Other Similar Conditions?
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Pena Shokeir Syndrome - Is characterized by low fetal movements and growth.
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Patau Syndrome - A rare genetic disorder characterized by an extra copy of the 13th chromosome.
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Distal Arthrogryposis Type 1 - Characterized by restricted joint deformities and movements.
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CHARGE Syndrome - This is an abbreviation of several other conditions, such as: C- Coloboma H- Heart diseases, A- Atresia choanae, R- growth Retardation, G- Genital abnormalities E- Ear abnormalities. A disorder that affects several body parts, including the heart, kidney, genital, and ear abnormalities.
How Can Trisomy 18 Be Managed?
As trisomy 18 is a genetic condition, there is no permanent cure for this condition. The causes of death in trisomy 18 are a cardiac and respiratory failure. Most of the cases are stillbirths, or they die soon after birth.
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Neonatal Intensive Care Unit - As these babies suffer from cardiac and respiratory failure, neonatal resuscitation efforts are made to restore cardiac function. They are performed in the delivery room itself. In the earlier days, resuscitation was not advised due to the low success rate. However, recently the American Academy of Pediatrics has advised resuscitation procedures.
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Feeding - These babies will have a low muscle tone and poor suckling ability; hence feeding is initiated through a nasogastric tube, and gastrostomy feeding is advised.
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Cardiac Problems - Medications such as Digoxin and Diuretics are advised to overcome cardiac failure. In severe congenital defects, corrective surgeries may be recommended.
What Is the Outcome of the Treatment?
The success rate is poor, despite the best medical aid. Nearly 40 % of babies die during labor itself. Many are delivered as preterm babies, and their survival rate is between three to 14 days. About 60 to 70 % survive only up to a week, 20 to 40 % for a month, ten percent for a year, and less than five to ten percent for more than a year. Female babies will generally have a higher survival rate than male babies.
Conclusion:
As trisomy 18 is a genetic condition, there is no cure or definite mode of treatment. Only palliative treatments provide symptomatic relief. This condition can be diagnosed during prenatal screening, so it is essential to take all the necessary tests to identify the defect. Parents and family members of children with trisomy 18 should be given psychological counseling so that they can mentally stay strong and also help the survivors of trisomy 18.
